The use of portable personal communication devices, such as cellular phones and pagers, has increased dramatically in recent years. Additionally, the use of portable navigational devices, such as Satellite Positioning System (SPS) receivers, has increased as these devices have become more widely available. Recent technological developments have allowed the combination of SPS receivers and communication systems in integrated units, such as a combination SPS receiver and cellular phone unit. Such combined devices have many applications such as personal security, emergency response, vehicle tracking, and inventory control. Some combination units combine separate SPS receivers and communication systems using suitable electronic interfaces. Others use shared circuitry and packaging. These combined units feature the convenience advantages afforded by common housings and integrated user interfaces. However, such combined units may also exhibit certain shortcomings, such as increased power consumption and reduced performance.
One marked disadvantage inherent in many combined SPS and communication devices is the decreased performance of the SPS receiver section of the combined unit. A common cause for this decreased performance is signal interference between the communication and SPS receiver stages. For example, in a combination cellular phone/SPS receiver, a cellular transmissions from the cellular telephone stage generate strong interference which can reduce the performance of the GPS receiver.
Current approaches to overcoming the cross-interference between the communication and SPS stages involve the use of complicated filters or high dynamic range circuits in the front-end section of the SPS receiver to limit the in-band interference to acceptable ranges. These approaches, however, require the use of complex additional circuitry which can increase the cost and power consumption of the combined unit. For example, one method of reducing the cross-coupling in a combination cell phone/SPS receiver is to use several bandpass filters in the RF front end of the SPS transmitter to eliminate the radio frequency (RF) interference from the cellular transmitter. However, there are several problems with this approach. First, several filters may be required to provide adequate reduction of the signal energy coupled into the SPS receiver RF circuitry from the cellular transmitter. This may increase cost and size of the combined unit. Secondly, the use of filters increases the noise figure of the SPS receiver, making it less sensitive to the satellite navigational signals.
It is therefore desirable to provide a system that reduces the cross-interference between the SPS and communication sections of a combined SPS/communication receiver.
It is further desirable to provide a system that improves the SPS reception performance in a combined SPS/communication receiver without adversely impacting the cost and sensitivity characteristics of the SPS receiver.